Many people were taught that they shouldn't end a sentence with a preposition. Today, however, most language experts don't abide by this "rule"—it's often called a myth. (Read more at Ending a Sentence with a Preposition.)

However, after I posted the article about ending sentences with prepositions, I was surprised by how many people asked if it is OK to start a sentence with a preposition. I've never heard a rule forbidding that practice.

Prepositional Phrases at the Beginning of a Sentence

Prepositional phrases at the beginning of sentences are common and grammatically correct. Consider these examples:

On the other hand, Bobby likes strawberries.

After soccer, we go out for pizza.

By noon, all the runners should be finished.

Over spring break, Shondra broke up with Lance.

Commas After Prepositional Phrases at the Beginning of a Sentence

When you start a sentence with a prepositional phrase, it's usually a good idea to put a comma after it (as in the examples above). In general, the longer the prepositional phrase, the more you need the comma. For example, the Purdue Online Writing Lab says a comma is required after introductory prepositional phrases that are longer than four words.